The major aim of this study is to provide a detailed understanding of attempts to stop or reduce marijuana use that can be used to develop better behavioral treatments for marijuana dependence. The application will provide a prospective description of attempts by dependent adult marijuana users to stop or reduce their marijuana use in a real-world setting. Although prospective, natural history studies describing attempts to stop or reduce alcohol, heroin and tobacco use have proved useful, we know of no such study among adult marijuana users. A pilot study will develop measures and assess compliance with our procedures. The main study will recruit 200 daily, adult marijuana smokers who plan to quit or reduce in the near future. Participants will call an Interactive Voice Recording (IVR) system daily for 3 months to report marijuana use, intentions to change marijuana use, quit/reduction attempts, and events that might increase or decrease the probability of initiating a quit/reduction attempt or the success of an attempt. Participants will be called weekly to obtain more detailed measures such as other drug use, self-efficacy, psychiatric/medical symptoms, and treatment seeking. Phone follow-ups at 4, 5 and 6 months will track marijuana and other drug use and dependence/abuse. PUBLIC HEALTH RELEVANCE: Knowing what motivates marijuana users to try to stop or reduce, and knowing which strategies for stopping marijuana use are successful, can help develop treatments for marijuana dependence. For example, if stopping tobacco when trying to stop marijuana decreases the chance of stopping marijuana, then smoking cessation should be done after stopping marijuana. Or if reducing marijuana often leads to cessation, then convincing those not interested in quitting marijuana to first try reducing may be helpful.